Fungal spore 'death clouds' key in gypsy moth fight

A gypsy moth caterpillar that has died in the forest canopy remains attached to a branch while infective fungal spores multiply within the cadaver.

A fungus known to decimate populations of gypsy moths creates “death clouds” of spores that can travel more than 40 miles to potentially infect populations of invasive moths, according to a new Cornell study.

That’s good news as gypsy moth (Lymantria dispar) caterpillars ravage the leaves of forest trees – especially oak and aspen – decimating forests, orchards and properties across the northeastern United States. In 2016, gypsy moth caterpillars ate the leaves off 350,000 acres of forest plants in Massachusetts alone.

The study, published online Aug. 17 in the journal Applied and Environmental Microbiology, describes a new method for tracking the geographic range of this airborne insect pathogenic fungus from areas of a disease outbreak.

Better understanding of the distances these killer spores travel could help researchers correlate the fungus’ range with weather patterns to better predict how bad gypsy moth damage will be in a given year.

“One spore can infect a gypsy moth caterpillar and kill the insect, and after it’s dead, the fungus can use the body to make one million more spores. An insect is a huge source of nutrients for making spores,” said Ann Hajek, professor of entomology and a co-author of the paper. Tonya Bittner, a postdoctoral associate in Hajek’s lab, is the paper’s first author.

The fungal pathogen (Entomophaga maimaiga) first appeared in New England in 1989 and only infects gypsy moths. The pollen-sized spores stick to caterpillars when they walk over them. Once attached, a spore uses enzymes to create a hole and enter the caterpillar’s body, where a cloaking mechanism allows the fungus to remain undetected by the moth’s defenses. Over four to six days, the fungus multiplies and then kills the host, after which new spores are literally shot from the cadaver into the air, where they become windborne.

From May through June, when gypsy moth caterpillars are feeding and before they pupate, the fungal pathogen can run through up to nine infection cycles, while the numbers of infections increase dramatically. During the study, the researchers found the peak caterpillar death rate due to E. maimaiga reached 86 percent, meaning that if you found 100 caterpillars munching leaves that day, 86 of them would die within the week.

In the past, researchers studied the airborne spores by collecting air samples on a transparent surface and studying particles under a microscope, a time-consuming and potentially inaccurate process, Bittner said.

The new method makes use of quantitative polymerase chain reaction (PCR), a standard method for quantifying RNA and DNA. The researchers designed a trap, a chamber with a hole on the top. 

“Whatever is falling in the air can fall into that hole,” Bittner said. A cup in the bottom of the trap contains a buffer that prevents the spores from germinating but preserves each spore’s DNA.

Back in the lab, the researchers filtered the contents of each trap for pollen-sized particles, then measured the amount of E. maimaiga DNA in each sample using quantitative PCR. “We found there was a correlation where if the trap was closer to a defoliated area, it had more spores, and further away it had fewer spores,” Bittner said. “We did detect spores in a trap that was 70 kilometers [43.5 miles] from a defoliated area.”

Andrew Liebhold and Harold Thistle, both United States Department of Agriculture (USDA) researchers, are co-authors on the paper.

The study was funded by the USDA and the Gypsy Moth Slow the Spread Foundation.

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Joe Schwartz